The present invention relates to an optical pickup adapted to control the direction of a light beam by means of a galvano-mirror (mirror-type deflecting device), for example.
Tracking control is effected by using a galvano-mirror in some of optical devices, such as information recording/reproducing devices for recording on and/or reproducing information from optical storage media, including an magneto-optic disk drive, write-once disk drive, phase-change disk drive, CD-ROM, DVD, optical card, etc. In carrying out the tracking control in the optical devices of this type, the angle of incidence of a light beam upon an objective lens is changed by controlling the inclination of the reflected beam by means of the galvano-mirror, and a beam spot that is formed on a storage medium by means of the objective lens is finely moved in the tracking direction.
If the reflected light beam is only inclined by means of the galvano-mirror, however, the light beam incident upon the objective lens is shifted in the direction perpendicular to its optical axis, so that the intensity distribution of light incident upon the objective lens is deflected inevitably. Thus, the beam spot may be deformed or tracking error signals may be subject to offsets.
In order to solve this problem, an arrangement is proposed in which a condensing or imaging lens is interposed between the objective lens and the galvano-mirror so that the reflective surface of the galvano-mirror and the front or condenser-side focus of the objective lens are in conjugate relation. In other words, there is a proposal to provide a condensing lens that associates the reflective surface of the galvano-mirror and the front focus of the objective lens in conjugate relative positions.
According to this arrangement, the reflected light beam is always incident upon the objective lens via its front focus without regard to the tilt of the reflective surface of the galvano-mirror. In consequence, the beam spot formed on the storage medium by the objective lens moves in accordance with the tilt of the reflective surface of the galvano-mirror.
If the focal length is deviated from a set value due to a workmanship error in the condensing lens, even in this arrangement, however, the conjugate relation between the reflective surface of the galvano-mirror and the front focus of the objective lens is broken off. This conjugate relation can be also broken off due to mechanical errors in carriages or the like that carry the galvano-mirror, objective lens, and condensing lens.
Thus, the beam spot may be deformed or tracking error signals may be subject to offsets, so that accurate recording and reproduction cannot be achieved.